In the field of telecommunications, duplex, or “two-way,” communication channels are commonly used. A full-duplex channel is a channel which can carry information in outgoing and incoming directions at one time. Several duplexing methods are commonly used for separating forward and reverse communication channels on the same physical communications medium.
Time division duplexing (TDD), for example, is commonly used to separate a telecommunication device's outgoing and incoming signals. Time division duplexing is particularly effective when uplink and downlink data speed is variable. For example, as the amount of uplink data increases, more bandwidth can be allocated to handle that data, and as it shrinks, less bandwidth can be allocated to handle that data.
One common problem associated with TDD communications equipment, such as TDD handsets, is electromagnetic interference (EMI), also known as radio frequency interference (RFD). Because TDD communications switches from transmit to receive mode at a high rate, this switching noise may cause RFI. RFI is any electromagnetic disturbance that interrupts, obstructs, or otherwise degrades or limits the effective performance of electronic or electrical equipment. RFI may be induced intentionally or unintentionally, as a result of unauthentic emissions and responses, intermodulation products, and other causes. TDD handset transmissions are often degraded by RFI.
Several attempts have been made to reduce or eliminate RFI associated with TDD handset transmissions. For example, passive filters, such as LC filters (i.e., inductance/capacitance filters), have been implemented on printed circuit boards (PCBs) used in TDD handsets to reduce RFI. These LC filters, however, are generally expensive, and typically occupy a significant portion of the available surface area on a PCB. Furthermore, an LC filter typically includes a bared lead wire that can reduce the ability of the LC filter to attenuate RFI.
Ring-structured filter devices have also been implemented on PCBs in TDD handsets to reduce RFI. In certain mediums, such as those typically used to construct PCBs, however, ring-structured devices often have a decreased effect, which leads to reduced RFI attenuation. For example, in a typical 4 mm×2 mm PCB, a ring-structured filter device can generally attenuate the RFI of a 900 MHz transmission (i.e., a standard GSM transmission frequency) by approximately only 10 dB.
While this degree of RFI attenuation is somewhat effective, several decibals of “noise” may, in some cases, still be present in the transmission, which can obstruct or degrade the transmission. As a result, the transmission may be unclear, and may not be decipherable in its entirety, or at all, by the intended recipient. Thus, a need exists for a filter that can be implemented on a PCB in a TDD handset, or similar device, to better attenuate RFI, while occupying a minimal amount of PCB surface area.